Pulse radiolysis of 4-tert-butyl-1,2-dihydroxybenzene and 4-tert-butyl-1

Helen W. Richter. J. Phys. Chem. , 1979, 83 (9), pp 1123–1129. DOI: 10.1021/j100472a003. Publication Date: May 1979. ACS Legacy Archive. Cite this:J...
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OH Reaction with 4-tert-Butyl-l,2-dihydroxybenzene

The Journal of Physical Chemistry, Vol. 83, No. 9, 1979

1123

Pulse Radliolysis of 4- ferf -Butyl- 1,2-dihydroxybenzene and 4- ferf -Butyl-l,2-quinone Helen W. Richtert Department of Chemistry, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 152 13, Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, and Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973 (Received September 18, 1978) Public(ation costs assisted ,by the Radiation Laboratory of the University of Notre Dame

A study has been made in aqueous solution of the radicals produced by hydroxyl radical attack on 4-tertbutyl-l,2-dihydroxybenzene,and by attack of solvated electrons and hydrogen atoms on 4-tert-butyl-l,2-quinone by the technique of pulse radiolysis using optical absorption for the detection of transients. Reaction sates with primary radicals were determined. The spectrum of the semiquinone radical, obtained in acid solution by reaction of OH with the hydroquinone, has absorption maxima at 290 and 390 nm, with extinction coefficients of about 7700 and 1860 M-' crn-l, respectively. The spectrum of the semiquinone anion, obtained by reduction of the quinone in neutral solution, shows a maximum at 313 nm with a shoulder at 350 nm, having extinction coefficients of about 1 2 200 and 2400 M-' cm-l, respectively. The pK, of the semiquinone radical is 5.2. The absorbance at 310 nm produced by the reaction of hydroxyl radicals with 4-tert-butyl-1,2-dihydroxybenzene decays by second-order kinetics below pH 7.5. At pH 3,2kb, = 9.0 X lo8 M-' s-l was obtained for the bimolecular reaction of two semiquinone radicals. As the pH is increased, the rate of second-order decay decreases dramatically, the computed rate constant for bimolecular reaction of two radical anion molecules being 2kb, I4 :r( lo6 M-' s?. The initial radical spectra and absorbance decay kinetics found in alkaline solutions differ from that in acid and neutral solution, absorbance decay proceeding mainly by first-order kinetics.

Introduction The hydroxyl radical is known to react with aromatic compounds preferen tially by an addition reaction yielding hydroxycyclohexadienyl radicals.' Uncatalyzed and acid-catalyzed water elimination reactions from these OH adducts have been reported for phenols,2 methylated benzene^,^ p-dihyclr~xybenzene,~ and 3,4-dihydroxyt01uene.~ Iit has been demonstrated clearly for p-dihydroxybenzene that the product of the water loss reaction is a semiquinone or semiquinone anion r a d i ~ a l .Recently, ~ data on the radicals produced by OH attack on o-dihydroxybenzenes have been reported for epinephrine6-8 and 3,4-dihydro~ytoluene.~ The present report presents optical absorption spectra, radical decay kinetics, and behavior with p H variation of radicals resulting from the OH attack on 4-tert-butyl-l,2-dihydroxybenzene (l),and attack of eaq-and H on 4-tert-butyl-1,2-quinone (2). The major path d reaction of hydroxyl radical with 1 is expected to be (1)addition to the benzene ring of the parent compound to give a trihydroxycyclohexadienyl radical (Rl), (2) loss of water from this radical, possibly in a two-step process, yielding a semiquinone radical (R4) in equilibrium with its anion (R5), and (3) bimolecular reaction of the semiquinone yielding the hydroquinone and quinone (Scheme I). Cation radicals similar to R3 have been observed to be produced from OH adducts of methylated benzenesag (Note that R2, R4, and R5 represent all the configurations available to the radical.) Solvated electrons and hydrogen atoms should reduce the quinone4 (Scheme 11). In the present investigation it is shown that in neutral and acid solution, with very low dose pulses, these simple schemes describe the major part of the chemistry occurring; however, iin alkaline solution or with higher doses other reaction modes become dominant. Experimental Section Methods and Materials. The pulse radiolysis facilities used were (11) the van de Graaff facility of Brookhaven National LaboratorieslO and ( 2 ) the linac facility of the Current adldress, Carnegie-Mellow University.

Scheme I

1

R2

R1

1

I

( Z b ) -.OH-

( 2 ~ )-OH-

-Hf

(2d)

HO

R3

., R4

R5

Scheme I1 (4)

fH'

2 + H-+R4-H+

(5)

R5 c2+

eag-

Radiation Laboratory of the University of Notre Dame." At facility (1) a multiple-pass cell with an effective pathlength of 6 cm was used. Digitized absorbance data from individual pulses were recorded on magnetic tape and kinetic analyses were made by computer fitting of the curves. The doses were measured with the Fricke dosimeter using G(Fe3+)= 15.6 and 4Fe3+) = 2200 M-' cm-' a t 305 nm. The radiolysis cell was flushed with nitrous oxide for 1 min prior to filling to eliminate effects due to traces of oxygen, which were quite strong. Fresh solution was used for each pulse. At facility (2) a single-pass flow system with a 1or 0.5 cm cell pathlength was used. For kinetic and spectral analyses the digitized data from several pulses were averaged.12 For dose measurements the thiocyanate dosimeter was used, assuming t[ (SCN),-]

0022-3654/79/2083-1123$01.00/00 1979 American Chemical Society

1124

Helen W. Richter

The Journal of Physical Chemistry, Vol. 83, No. 9, 1979

is 7600 M-' cm-' a t 480 nm and G[(SCN),-] = 6.0 in a NzO-saturated solution containing 10 mM SCN-. All experiments were carried out at room temperature. The doses employed were between 40 and 600 rd per pulse. In N20-saturated solutions e, - (G, - = 2.8) are converted to hydroxyl radicals [e,; + N z b kz+ OH- + OH, k = 8.7 X lo9 M-' s-l13] or hydrogen atoms [eaq-+ H+ H, k = (2.3-2.4) X loio M-' s-113] so that only OH radicals (GOH = 3.2) and H atoms (GH = 0.5) remain to react with solute. At pH 3.5, the lowest pH employed, 96% of the e , i escape neutralization, giving G(0H) = 5.9. Therefore, with little loss in accuracy, it can be assumed that in the experiments with pH 13.5, G(OH) = G,- + GOH = 6.0 and G(H) = G H = 0.5, Le., H atoms constitute 8% of the reactive radical population. For the computation of the extinction coefficients of the products of reaction with OH, it was assumed that G(product) = G(0H) = 6.0, neglecting the contribution of the H atom. Since H atoms are reactive toward aromatic compounds, the reported spectra do contain a contribution from the H atom products. When correction of spectra for bleaching of 1 was required, G(-1) = G(0H) G(H) = 6.5 was used. In the experiments with Nz-saturated solutions containing tert-butyl alcohol, it was assumed that G(product) = G(-2) = GH + Geq- = 3.3. The spectra of 1 and 2 used to correct transient spectra for bleaching of 1 and 2 were taken in N2-saturated solutions with pH adjusted to that of the radiation experiment. 4-tert-Butyl-1,2-dihydroxybenzene was obtained from Aldrich Chemical Co. and was purified by sublimation in was prepared vacuo a t 60 "C. 4-tert-Butyl-1,2-quinone from purified parent compound by oxidation with AgZOi4 and was passed through an anhydrous sodium sulfate bed to increase ~tabi1ity.l~ Potassium phosphate (monobasic and dibasic) and potassium hydroxide were reagent grade from Baker Chemical Co. Nitrous oxide was treated for oxygen removal and water saturation by passing through a solution of VI1 in 1 N H2SO4(prepared by adding Hg-Zn moss amalgam to a 0.1 M vanadyl sulfate solution).

-

-

+

Results and Discussion A pulse radiolysis study of nitrous oxide saturated aqueous solutions of 1 with the pH between 3.0 and 10.5 was carried out using optical absorption for the detection of transients. Under these conditions, greater than 88% of the primary radicals are available as OH radicals for reaction with 1. Reactions corresponding to the addition of OH to 1, reaction 1 of Scheme I, and to the loss of water from the adduct, reaction 2 of Scheme I, were seen under certain conditions. It was observed that the radical present following the water-loss reaction existed in an acid and base form, corresponding to R4 and R5, and that the species decayed by second-order kinetics over a wide range of conditions, As the pH was increased above 7.5, the observed changes no longer corresponded to Scheme I. The reduction of the quinone 2 by H atoms and solvated electrons at pH 7 was examined to obtain the spectrum of R5 for comparison with the spectrum obtained by oxidation of 1 with OH radicals. The Initial Reaction between OH and 4-tert-Butyl1,&dihydroxybenzene. When a NzO-saturated solution of 1 a t pH 7.0 was pulse radiolyzed, three reactions were observed on different time ranges: (i) an initial absorbance growth a t 338 nm, first order in radical concentration, complete on the 20 ps full-scale range, (ii) a first-order absorbance growth at 313 nm, complete on the 200 ps full-scale range, and (iii) a second-order absorbance decay at 313 nm, seen at t > 200 ps (kinetic order characterization based on computer fitting of growth or decay curves and response to dose variation). Reaction i is assigned to the

250

350

300

400

X/nm

Figure 1. (0)Spectrum of transients present 7 ps (average of 6-8 ps data) after the end of the pulse in a nitrous oxide saturated solution containing 0.1 1 mM of 1 and 5 mM phosphate buffer at pH 6.97. The dose was -300 rd per pulse. ( 0 )Spectrum of the OH adduct of 1, R1, obtained by correcting the 7-ps spectrum for a 19% component of semiquinone anion (spectrum (O), Figure 3). Both spectra are corrected for bleaching of 1.

pseudo-first-order reaction of OH with 1, reaction 1 of Scheme I. Kinetic measurements were made at 338 nm since this is an isobestic wavelength for the products of reactions i and ii (at 313 nm the absorbance is still growing in 20 p s after the pulse). First-order dependence of the rate of reaction i on [ 11 was demonstrated, confirming its assignment: hi = 6.5 X lo9 M-' ([l] = 1.1X M), and kl = 7.6 X lo9M-I s-l ([l] = 5.1 X 10" M), where doses of about 300 rd were used. The value obtained with the concentrated solution is less accurate, since the half-life of reaction is only 1 ys (five data points from the absorbance digitizing system). The G(c) obtained with the two [ l ] were within 10% of each other, indicating nearly complete scavenging of OH by 1 (when [ l ] = 5 X M, the calculated loss of OH via recombination is 2.8% with the present experimental conditions). The average rate constant is lower than the values which have been observed for substituted phenolsi6 (1.0-2.2 X 10'O M-l s- ) , 3,4-dihydroxytoluene5 (1.6 X 1O1O M-I s-l), and epinephrine8 (2.2 X loio M-l s-l) Spectrum 0; the Hydroxyl Radical Adduct of 1 . A NzO-saturated solution containing 0.11 mM of 1 and 5 mM phosphate buffer at pH 7.0 was pulse radiolyzed with doses of 300 rd per pulse. The spectrum of the transients present 7 p s after the end of the pulse (average of 6-8 ps data from several pulses) was determined. Under the conditions used, the reaction with OH would be 99% complete; however, about 19% of the adduct radicals formed will have undergone the water-loss reaction, since k2 I3.5 X lo4 s-l, as will be discussed in the next section. The 7-ps spectrum, corrected for bleaching of 1, is given in Figure 1. Assuming that this spectrum consists 81% of the adduct and 19% of semiquinone anion (see later sections), the spectrum of the adduct can be extracted and is given in Figure 1 (at times as early at 7 vs, possible bimolecular reactions of the radicals R1 and R5 have no significant effects). The computed adduct spectrum has a broad maximum at 310 nm with an extinction coefficient of 4300 M-l cm-l; however, since the absorption peak seen is near the 313-nm absorption maximum of R5,the possibility remains that there is still a contribution from R5 in the computed spectrum. This situation would occur if some portion of the OH radicals react with 1 by direct abstraction of an H atom. Similar broad absorption spectra

OH Reaction with 4-fert-Butyl-l,2-dihydroxybenzene

v--m

The Journal of Physical Chemistry, Vol. 83, No. 9, 1979 12

I

"

"

1125

1

A /nrn

2t

\w \

PH Flgure 2. (1)ph,: of the semiquinone of 1 determined by measurement of ( 0 )the absorbance at 310 nm extrapolated to the end of the pulse from decay d a h in N,O-saturated solutions of 1, and (B) the plateau absorbance at 310 nm resulting from reduction of 2 in nitrogen-saturated solution containing 0.5 M tert-butyl alcohol. Doses were -200 rd per pulse. Each point is the average of several pulses, fresh solution being used for each pulse. [l] = 1 mM, with phosphate buffer = 10 mM; [2] = 0.1 mM, with phosphate buffer = 6 mM. The smooth curve is the computed absorbance coefficient assuming pK, = 5.2,4R4) = 3850 M-' cm'', and 4R5) = 11600 M-l cm-'. Abscissa scale given on left side of figure: (2)(0)pH dependence of the bimolecular decay at 310 nm of the radical produced by OH attack on 1. The smooth line was computed using pK, = 5.2,4R4) = 3850 M-' cm-', e(R5) = 11600 M-' cm-', 42) = 640,k3a = 4.5 X lo8 M-' s-' k3b = 6.2 X lo8 M-' s-' , and (solid line) k,, = 2 X 106 M-' s" , or (dashed line) k,, = 0. The (experimental conditions are the same as used in the pK, determinaiion. Abscissa scale given on right side of figure.

have been observed for the adducts of substituted phenols.16 The Water-Loss Reaction of the OH Adduct. The second reaction observed in the pulse radiolysis of N20saturated solutions of 1 at pH 7.0, a first-order growth in absorbance iit 313 rim complete on the 2 0 0 - ~ sfull-scale range (reaction ii of the second preceding section), is assigned to reaction 2 of Scheme I, the loss of water from the OH adduct R1. The rate of loss of H 2 0 from R1 is relatively slow at pH 7; as a result of this, second-order absorbance losses (via reaction 3 of Scheme I and probably by bimolecular reactions of R1) iinterfere with the determination of the reaction rate constant. When the dose per pulse was lowered to 60 rd (14 experiments averaged), the extinction coefficient calculated from the plateau in absorbance following reaction 2 was only 10 400 M-l cm-l, compared with 12 000 M-l cm-I obtained for the extinction coefficient of R5 by the reduction of 2 a t pH 7, described in a later section ( i t is assumed that the equilibrium between R4 and R5 is established instantaneously on the time scale of the experiment, and, ais will be shown in the next section, R5 is the predominant species a t pH 7). Thus, even at very low dose competition with second-order reactions is not eliminated and a significant quantity of R5 has deca:yed via reaction 3 before reaction 2 has gone to completion. In addition, the first-order kinetic fits of the data, which appear to be very good, give a maximum ' value for the rate coinstant: k2 5 3.5 X lo4 8.Adams and Michael4 observed a minimum rate constant for loss of H20 from the O H adduct of p-dihydroxybenzene of 4.6 X lo4 s-l a t pH 7.2.

Flgure 3. (1)Spectra of the transients produced from the reaction of OH with 1 which are decaying by secmd-order kinetics 100 @safter the pulse end at pH 3.5 (0, B) and at pH 7.3 (A,A). Nitrous oxide saturated solutions contained 0.4mM of 1 and 10 mM phosphate buffer, and doses of 200-400 (pH 3.5)or 300-600 (pH 7.3)rd per pulse were used; for data near absorption maximum of 1 (pH 3.5),solutions contained 0.1 mM of 1 and doses were 100 rd per pulse. Absorbance data were obtained by extrapolating decay data to the pulse end. The solid points are the zero dose values obtained from absorbance vs. dose plots. Spectra are corrected for bleaching of 1 at wavelengths Spectrum of the transient present 70 ps (average below 300 nm: (2)(0) of 60-80-w~ data) after the pulse end in a nitrogen-saturated solution containing 0.1 mM of 2,0.5 M fert-butyl alcohol, and 5 mM phosphate buffer at pH 7.1. The dose was 300 rd per pulse. Spectrum corrected for bleaching of 2.

-

pK, of the Semiquinone Radical. The species present following reaction 2 of Scheme I decay by second-order kinetics at pH below 7.5, corresponding to reaction 3 of Scheme I (and reaction iii seen at pH 7 in the third preceding section). The extinction coefficient obtained by extrapolating the second-order absorbance decay curves at 310 nm back to the pulse end are a function of pH, as shown in Figure 2. The smooth line is computed by assuming that the pH dependence of the extrapolated absorbance results from the acid dissociation process R4 + R5 H+, where the extinction coefficient of R4 is 3850 M-l cm-l and of R5 is 11600 M-l cm-l. The acid dissociation constant giving the best fit to the data is pK, = 5.2. This value can be compared with pK, = 4.0 obtained for the semiquinone of p-dihydr~xybenzene~ and pK, = 4.5 for the semiquinone derived from 3,4-dihydro~ytoluene.~ The large difference between the pK, of R4 and the pdihydroxybenzene radical seems reasonable, but one would not expect the substitution of a tert-butyl group for a methyl group to result in a pK, difference of 0.7. Spectrum of the Semiquinone Radical. The spectrum of the transient decaying by second-order kinetics a t pH 3.5 was obtained by extrapolating the absorbance decay curve back to the end of the pulse, Figure 3. Doses between 200 and 400 rd per pulse were used. A t 300 nm the extinction coefficient obtained from an OD vs. dose plot was only slightly higher than that obtained in the series of spectral determinations. At pH 3.5 the water-loss reaction is more rapid than at pH 7. The spectrum has maxima a t 290 and 390 nm, with extinction coefficients of 7700 and 1850 M-l cm-l, respectively (the spectrum is corrected for bleaching of the parent compound). This transient is presumably the semiquinone radical R4. Spectrum of the Semiquinone Anion Radical. When the pH was increased to 7.3 and the spectrum of the transient decaying by second-order kinetics was determined as described in the preceding paragraph, the spectrum given in Figure 3 was obtained (doses of 250-500

+

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The Journal of Physical Chemistry, Vol. 83, No. 9, 1979

rd per pulse); however, when the extinction coefficient at 310 nm was determined by averaging several experiments a t very low dose (40-50 rd per pulse), the value obtained was 18% higher than that from the series of spectral measurements. This result indicates that bimolecular decay of the OH adduct R1 can compete effectively with the HzO loss reaction, since even if reaction 2 is slow with respect to the bimolecular decay of R5 the extinction coefficients determined by extrapolation of the decay curve to the end of the pulse should not be dose sensitive if the decay is purely R5 + R5 (or R5 + R4). Thus, the spectrum of the semiquinone anion radical is not readily obtained when R1 is an intermediate in the production of the anion. A nitrogen-saturated solution of 2 containing 0.5 M of tert-butyl alcohol at pH 7.1 was pulse radiolyzed. Under these conditions, only H and e,; are available to react with 2, producing R4 and R5 directly, as shown in Scheme 11. Two reactions resulting in increasing absorbance at 313 nm were observed, requiring about 1.6 and 60 gs, respectively, for completion. The two processes can be assigned to the addition of H and e,; to 2, reactions 4 and 5 of Scheme 11,respectively, as the ratio of the absorbances produced corresponds to the ratio of the yields of ea; and H. The rate constant for reaction of 2 with ea; was roughly 1 X 10'O M-l s-l, and with H was 7.8 X lo8 M-' s-l. The resulting absorbance plateau was stable until at least 160 ps after the pulse when the dose was 300 rd per pulse. The absorption spectrum at 70 ps is given in Figure 3, corrected for bleaching of 2. Assuming the establishment of the equilibrium between R4 and R5 is instantaneous on this time scale, the spectrum can be assigned unambiguously to the semiquinone anion R5, the predominant species at this pH. The spectrum has a maximum at 313 nm and a shoulder at 350 nm, having extinction coefficients of 12 200 and 2400 M-' cm-l, respectively. While the assignment of the spectrum obtained from the reduction of 2 in neutral solution to the semiquinone anion is unambiguous, the assignment of the spectrum obtained from the oxidation of 1 by OH is not. However, the extinction coefficient a t 310 nm obtained at very low dose from the pH 7.3 experiments with 1 matches that obtained from reduction of 2, and the peak shapes from the two experiments are similar, so that it is reasonable to assume that the major portion of the OH radicals leads to the production of R5 at sufficiently low does. Second-Order Decay of the Transient. Over a wide pH range the absorbance at 310 nm produced in NzO saturated solution decays by second-order kinetics. For a radical with an acid proton, such as R4, the bimolecular decay of absorbance via disproportionation, reaction 3 of Scheme I, is the result of three reactions: R4 R4 -,1 + 2 (34

+

+H+

R4+R5-1+2 R5

+ R5

-+ +2H+

1

(3b) 2

(34

The observed absorbance decay at a given pH has the same form as a simple bimolecular decay of an absorbing radical to an absorbing product: -d(OD)/dt = 2(OD - OD,)2(K/E) (1) where, for the simple case of

kbi

2R P the constant KIE is given by

K/E = kbi(ER - 1/Z+)

Helen W. Richter

(eR and tp are the extinction coefficients of the reactant and product, respectively.) For the general case, the value of K / E is constant at a given pH:

K/E = {k3a(tR4- yzc2)+ yzk3b(tR4+ ER5 - c z ) ~ , / [ H ++] h3c(ER5 - 1/c2)K,2/[H+12)/~~R4 - '/2'?2 + ( t R 5 '/,E2)Ka/[H+112 tR4, cR5, and t2 are the extinction coefficients of R4, R5,and 2; it is assumed that c for 1 is 0 at the wavelength of the measurement. K, is the acid dissociation constant of R4. In sufficiently acid or alkaline solution, K / E reduces to the form of the simple case with kbi, tR,and tp equal to k3,, tR4, and t2 or k3c, em, and c2, respectively. The K/E terms obtained from a second-order fit, by eq I, of absorbance decay data from pH 3 to 7.5 are plotted in Figure 2. In acid pH a plateau is established, indicating that K / E is reduced to the simple form with hbi = h3,, so that k3, can be calculated directly: 2k3, = 9.0 X lo8 M-I s-l at pH 3. As the pH is increased to neutral, K / E decreases by a factor larger than 100. As the pH is increased above pH 7.5 the absorbance decay no longer appears as a straightforward second-order process. In experiments at pH 6.9 and 7.4 the decay data are no longer well-fit by second-order kinetics. The smooth line in Figure 2 was obtained using experimental values of h3,, tR4, tR5, t2 (640 M-' cm-l at 310 nm), and K,, and best-fit values of 2k3b and Zk3, (1.2 X lo9 and 4 X lo6 M-l s-', respectively). The shape of the calculated curve was very sensitive to the value chosen for 2k3bin the region above pH 4, the value giving agreement with the data being 2k3b = (1.2 f 0.1) X lo9 M-l s-'. The curve fitting was not very sensitive to k3c in this pH range, and it can be concluded only that k3c is at least a factor of 100 smaller than k3,, or 2h3, 5 4 X lo6 M-' s-', The insensitivity of the fit may be seen by comparing the smooth line with the dashed line where kk was set to zero (ionic strength effects on k3, were ignored in view of this insensitivity). The rate of the bimolecular decay of the negatively charged basic form of a radical is expected to be relatively slow due to charge repulsion. Reported values for 2k3, and 2k3, for p-dihydroxybenzene are 1.1X lo9 and 1.7 X lo8 M-' s-l , in pH 2 and neutral solution, re~pectively.~ The authors verified that the reaction characterized by k3cis a bimolecular reaction between species bearing identical unit charge by means of an ionic strength study. For 3,4-dihydroxytoluene the reported value for 2k3, is 5.8 X los M-' s-I (pH 3);5no data are given for k3,. The value obtained for hSain the present study is very close to the reported value for the semiquinone of p-dihydroxybenzene, but it is about 60% larger than the value from 3,4-dihydroxytoluene. One would have expected the k3, from the two o-dihydroxy compounds to be very close. The finding that k, is very small compared to the k3, for I, whereas k3, is only a factor of 7 smaller than k3, for p-dihydroxybenzene is indicative of the strong effect of charge repulsion in the bimolecular disproportionation reaction. With either orientation, reactions 3a and 3b do not require the approach of two negatively charged species. Rather, one radical simply abstracts a hydrogen atom from the other, and the orientation of hydroxyl groups in the parent compound has little effect on the reaction, as shown by the nearly equal values found for h3, with 1 and p-dihydroxybenzene (Scheme 111). When both radicals are charged, the reaction of the radicals with ortho orientation (A) is disfavored since the charged

(y;=---on .,

o ~ o - - - - - o y J 0-

-0

A

B

OH Reaction with 4-tert-ButyC1,2-dihydroxybenzene

The Journal of Physical Chemistry, Vol. 83, No. 9, 1979

Scheme I11

0

L 4

H 0,

12

20

40

60

1127

80

-

R4 7E

-

IO

-

2

2 7w

r-

1

1

*.:I-

" s0 x

d O

6-

00.

O

200

300

Dose / r a d

I

0.25

Dependence of the OD, extrapolated to the pulse end, of the species decaying by second order in N,O-saturated solutions upon absorbed dose: (0)pH 3.5, 300 nm, coordinate scales given on lower pH 7.3, 310 nm, coordinate scales given on upper and left axes; (0) and right axes. Solution conditions given in Figure 4. Flgure 5.

- 2.01

0.05

I O 1I A I 01 02

IO0

I

03 04 Dose / k r a d

05

Flgure 4. Dependence of decay rate coristants on dose at four pH values for the transients produced by OH attack on 1. (0)pH 3.5, K I E determined at 300 nm, [ 11 = 0.5 mM; (0, 0 ) pH 7.3, KIEdetermined 0 ) pH 8.9, at 305 and 310 nm, respectively, [ l ] = 0.5 mM; (0, first-order constants measured at 320 mi, [l]= 0.5 and 0.2 mM, respectively; (A)pH 10.0, first-order constants measured at 320 nm, [ 11 = 0.4 mM. All solutions were N,O-saturated and contained 10 mM phosphate buffer.

groups must approach closely for electron abstraction to occur while the radicals with para orientation (B) feel a relatively simall effect from the approaching negative charges since they me at opposite sides of the benzene ring. This explanation of the relative rate constants implies considerable localization of electron charge on the oxygen atoms. Conditions for Second-Order Transient Decay. Several experiments were performed to verify the second-order character of the absorbance decay of R4 and R5 and the conditions under which the decay is second order. A t pH 3.5, where the transient absorbance was monitored at 300 nm, the concentration of 1 was varied over a 20-fold range and no variation wi3s seen in the bimolecular decay constant. With a dose d 200 rd, concentrations of 1 of 20,49, 85, and 425 ILMgave rate constants (relative values) of 1.80, 1.69,1.75, and 1.75, respectively. The decay data were well fit by second-order kinetics equations. A variation of dose by a factor of 8, up to 450 rd, yielded no variation in the K I E determined, Figure 4. A plot of extrapolated initial optical absorbance vs. absorbed dose yields a straight line passing through the origin for doses of 260 rd or less, Figure 5. Thus, at pH 3.5 the transient decay is very well described as a second-order process. At p H 7.3, with the transient followed a t 310 nm, the second-order character of the decay is not straightforward. A plot of the extrapolated initial optical density vs. ab-

sorbed dose, Figure 5, yields a straight line intercepting the origin only up to doses of about 50 rd (Le., initial radical concentration of 0.3 pM). A plot of K / E (at 305 and 310 nm) vs. dose, Figure 4,shows that at doses greater than 250 rd this quantity is constant, but that below this level the computed K / E increase. When the pH is increased to 8.9 and transient absorbance is observed at 320 nm, a second-order decay is no longer seen. Instead, one finds a slow decay which is first order in radical concentration; however, as the dose is increased, the apparent first-order constant increases, Figure 4. The behavior of the apparent second-order constant at pH 7.3 and of the first-order constant at pH 8.9 with dose indicates that at these pH levels there is a first-order reaction path available to the product radicals from OH attack on 1, and one can conclude that the major portion of the radicals decay by a second-order mechanism only up to pH 7.3. Products of Second-Order Transient Decay. The spectra of the products of the second-order radical decay in the OH 1 experiments were determined at pH 3.5 and 7.3, Figure 6, where the extinction coefficients are computed assuming G(product) = 3.0. The product absorption at 400 nm was stable for at least several seconds. The spectra have broad absorption bands with maxima near 400 nm, which is characteristic of o-quinones. However, the spectra are not identical with the spectrum of 2, also shown in Figure 6, indicating that the bimolecular radical decay probably yields the quinone and one or more other products. On the basis of the absorption intensities at 400 nm, the yield of 2 at pH 3.5 is no more than 80%, and at pH 7.3 no more than 65% of that expected if reaction of OH radical with 1 proceeds by Scheme I. The observed, low values for the maximum yields of quinone can result if (i) the bimolecular reactions of the semiquinone and its anion have alternate modes not yielding 2, e.g., dimerization, (ii) bimolecular reactions of the OH adduct of 1 compete successfully with loss of H20 from the adduct, or (iii) the reaction of OH with 1 yields radicals other than R1,e.g., via abstraction of an H atom from the tert-butyl g r ~ u p . ~Products J~ of bimolecular reactions of R1 clearly contribute to the spectrum of pH 7.3 in view of the doses used (250-550 rd) and the dose sensitivity of the system

+

Helen W. Richter

The Journal of Physical Chemistry, Vol. 83, No. 9, 1979

1128

r

I

*/I'i

I

I

1 1

I

TABLE I: Reaction Rate Constants

reaction

rate constant

h/nm

7.8 X 1 O s a

313 313 338 338 313 313 313 310 310 310

H+2

e& + 2 OH- 1

.a :\

1 x 1O'O

7.6 X 6.5 X l o 9 a 7.8 x l o g a 6.6 X l o 9 a G3.5 X lo4 9.0 X 10" 1.2 i( l o 9 c ) d

OH t 1-,1 R 1 - H,O R4 R4 R4 t R 5 R 5 + R5

-

a 1

I

I

300

350

500

450

400 Xinm

Figure 6. Spectra of the products of second-order transient decay in N,O-saturated solutions at pH 3.5 ( 0 ,0 )and 7.3 (0,0 ) . Experimental conditions are as in Figure 4; doses for the pH 3.5 spectrum were 100-400 rd per pulse, and for the pH 7.3 spectrum 250-550 rd. pH 7.3 spectrum is product present at 9 s; pH 3.5 spectrum is taken from 0.5 s absorption or final absorptions obtained from computer fitting of decay curves with full-scale times of 100 or 20 ms (experiments comparing computed and observed absorbances agreed very well). Smooth line is spectrum of 2 at pH 7.

"n-

a